Purpose: :
Previous work in our laboratory has shown that endogenous adenosinelevels suppress transmitter release from photoreceptors. Interestingly,adenosine levels are influenced by conditions of illuminationas well as by adenosine transporters. However, it is not knownhow adenosine levels are regulated in the outer retina. Thegoal of this study was to investigate the effect of light, adenosinetransporters, and the distribution of enzymes involved in theregulation of adenosine in the outer retina.

Methods: :
Experiments were performed in preparations from mice and larvaltiger salamanders. [Ca2+]i levels were measured from photoreceptorsusing either fura-2/AM or fluo-4/AM. Whole cell patch-clamprecordings were used to measure photoreceptor calcium currents(ICa) and light-evoked currents in second-order neurons to assesstransmitter release. Adenosine measurements were made usingadenosine biosensors with a potentiostat. Antibodies to nucleosidetriphosphate diphosphohydrolase-1 (NTPDase1), -2, and -3 wereused to map the localization of purine metabolism in the outerretina.

Conclusions: :
Our findings indicate that adenosine levels are tightly regulatedby conditions of illumination and ecto-nucleotidases in andaround photoreceptor terminals. These findings highlight theimportant and dynamic light-modulated nature of adenosine ontransmitter release in the retina and provide a novel way toregulate both the excitability and tonic release of L-glutamatefrom photoreceptors.